I have a project and I want to invite external user, which also has Atlassian/Jira license -->
billing is done on a site level, so if you grant access to your site for a user to access Jira, then he'll increase your bill. It doesn't matter if he is already paying a Jira license for another site.
So in your case, both companies will pay for this user.
Let me know if this helps,
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